With more and more states legalizing marijuana for both medical and recreational use, law enforcement officials have begun using tests to see if people are driving while high — the thing is, those tests are actually completely useless.

A study commissioned by AAA found "it's not possible to set a blood-test threshold for THC, the chemical in marijuana that makes people high, that can reliably determine impairment," the Associated Press reported Tuesday.

Six states, Colorado, Montana, Nevada, Ohio, Pennsylvania and Washington, currently test for THC levels to determine level of impairment. The problem with this, the AP reports, is that THC levels don't necessarily correlate with being impaired.

"Drivers with relatively high levels of THC in their systems might not be impaired," the AP reports, "especially if they are regular users, while others with relatively low levels may be unsafe behind the wheel."

THC levels can also decline rapidly in the time between getting pulled over and having a blood test done. Instead of testing for THC levels, the study recommends that officers screen drivers "for dozens of indicators of drug use, from pupil dilation and tongue color to behavior" and then back it up with by testing to see whether THC is present.